The present invention relates to fluidic angular rate sensors and more particularly to devices for detecting displacements of fluid jets.
In fluidic devices of various types, such as fluidic angular rate sensors, it is desirable to detect small displacements of fluid jets relative to a fixed axis and to convert the detected information to electrical signals. One technique for detecting such jet displacements is to heat a temperature sensitive wire resistor, centered on an axis defined by the undisplaced fluid jet, to a selected operating temperature and monitor wire resistance variations due to displacements of the fluid jet. However, this technique has several inherent disadvantages. For example, ambient temperature changes may be erroneously detected as jet displacement. Also, it is difficult to measure relatively small absolute wire temperature and resistance changes due to small displacements of the fluid jet. Moreover, it is impossible to determine the algebraic sign (direction) of the jet displacement solely by using this technique.
Sensitivity to ambient temperature changes and difficulties associated with measuring absolute wire temperature and resistance changes and with determining the displacement direction may be mitigated by using a two wire detector and measuring resistance differences between the two wires. According to this two wire technique a matched pair of hot wires is mounted in the fluid jet stream so that the wires are located symmetrically on either side of the axis defined by the center line of the undisplaced jet velocity profile and the differential resistance between the two wires is monitored. The nominally identical wires are equally cooled by an undisplaced jet so that their temperatures, and thus their resistances, are equal. Displacement of the fluid jet upsets the symmetry and unbalances the resistance.
Unfortunately, resistance unbalance in the two wire detector may also result from extraneous effects, such as ambient temperature changes, unless the wires are truly identical in characteristics such as geometry and composition. The precise wire matching needed to prevent false displacement indications due to such extraneous effects is difficult and costly to achieve. Also, it is difficult to maintain precise wire matching because of physical changes in the wires during the operating life of a sensor of which the wires are a part. These problems are of concern in manufacturing angular rate sensors which use the matched wire technique to detect displacements of a small gas jet subjected to forces such as rotation induced Coriolis forces.
According to the principles of the present invention an oscillating wire detector overcomes the difficulties of a two wire detector without incurring the disadvantages of a stationary one wire detector. The wire is oscillated through a fixed axis and in a plane generally perpendicular to the fluid jet stream. Change in average temperature or resistance variation of the oscillating wire as a function of wire position indicates the amount of displacement of the fluid jet relative to the fixed axis. Ambient temperature changes are effectively averaged out and relatively small temperature changes due to small jet displacements are detectable because only differences in temperature or resistance are measured.